ONI Alpha Site
Blowing up the Bridge
Start by turning left, walk towards the first switch and throw a nade into the closest tire on the ground so that you run over it and jump to get a nade boost. Activate first switch, head over exploded Scorpion tank to activate second. Head to watchtower to activate the explosives. Go to either side platform to obtain a nade. Two nade boosts can be routed in to get to the switches faster. If this is done, after activating the explosives there is time to go to both platforms to refil both nades before the door to the next section opens. A combination of Sniper, Spartan Laser or SMG can be used to clear the next waves. If using the sniper make sure to grab the ammo in the tower as well as on the floor at the top of the ramp. If you are planning on using all three weapons by weapon-swapping the third weapon as you walk make sure to pick up the sniper on the ground behind the barricades first and then collect the ammo from the one in the tower.
Hunter Wave Fight
While walking up the stairs to the green baracade you can use the Spartan Laser on the grunt turret so that it does not knock you out of scope. Hiding behind the baricade stops the Phantom turret from shooting you. Spartan Laser the first Hunter, move to the left stairs and line up a Spartan Laser collateral on the Brute exiting the Phantom and the second Hunter. Double nade the rest of the enemies on the stairs and head down the stairs. Look left and clear the 3 Grunts and 1 Brute and then turn around and check for survivors.
When killing the first Hunter start charging the Spartan Laser just as Mickey says the words "the wall" ("Covenant landing on the other side of the wall."). This sets up the timing for the second Spartan Laser shot. Immediately charge the second shot for the second Hunter and the Spartan Laser should go off just as the brute is landing. This minimises the chances that the brute moves and dodges the Laser or that he deploys a bubble shield.
You can use three weapons by weapon swapping a third weapon up the main stairs all the way to the left of the barricade. kill both hunters and the brute with two Spartan Laser shots and then immediately switch it with your 3rd weapon on the ground. Proceed to clear the remaining enemies with the Sniper and the SMG.
The last group of 3 grunts and 1 brute can be cleared with one Sniper clip if you hit a two or more enemies with 1 bullet. This can save a lot of time as you avoid the need to reload the Sniper and conserves ammo. Otherwise bringing the SMG and cleaning up by switching weapons to it (instead of reloading) would be the fastest backup. It can be easier to kill for the grunts first as killing the Brute first will usually cause them to panic and run in circles which makes them difficult to hit with the sniper.
The hill fight can be completed using many different combinations of Spartan Laser, Sniper, SMG or Pistol. The aim is to clear enemies as quickly as possible until a dialogue queue. Head behind the sand bags on the left side (facing the ONI building) to pick up a nade, there is also a pistol and SMG ammo here. Head to the center to collect a second nade. As the first phantom on the right (when facing down the hill) arrives, opens its doors and lowers start shooting the grunts while moving towards the stairs on the left. After killing the brute and grunts, move to the staircase on your left and throw two frags at the enemies feet in quick succession as they land on the stairs. Hopefully all the grunts and maybe a brute or two will die to the frags. Go left and throw a frag at the group of grunts that are jumping out of the phantom. If you have the Spartan Laser shoot the wraith. Head up the far side of the hill while also trying to kill the remaining brutes. Now circle back around to the other side. While traveling across shoot the second wraith if you have the Spartan Laser otherwise start shooting the remaining enemies. Focus on the group of 2 brutes and grunts on the far right side. Once you have cleared enough enemies Mickey will say "More Phantoms, look sharp". At this point a timer has started. Head up the hill and grab the sniper at the top, grab the rockets down the stairs and behind the pillar and wait for the doors to open.
Using the sniper the first group of 3 Grunts and 1 Brute can be cleared before the second group lands. When shooting the Wraiths position yourself front on so that you can shoot the drivers hatch to one shot the vehicle. Alternatively as the first wraith is landing charge the Spartan Laser to coincide with the Wraith landing and aim for the power core on the back. You will then have just enough time to turn and nade the group of grunts that jump out of the Phantom. As you move along walk over grunts to try and get plasma nade drops which can be used for quick kills on the brutes. Keep moving and leave brutes alive if you have to, don't go back. Priotise the grunts right up until the last group before turning back onto the brutes.
Head through the sliding doors as soon as they open, go through the 2nd set of doors and then turn towards Mickey's waypoint so that it is roughly in the centre of the screen and continue walking backwards towards the Turret. If your lucky Mickey will teleport as you arrive near the turret and the dialogue queue will start. If you are unlucky, or if you look away from Mickey he will teleport to the turret area but will walk back to his last location and then continue on his path back to the turret before the dialogue finally continues which can waste some time. Otherwise Mickey will walk his way to the turret which can sometimes be fast enough but is often quite slow.
Once the Sergeant starts saying "Alright men check these charges" give your sniper to the Sergeant, grab the turret and try pushing the crate of four nades up the stairs to the middle area behind the pillar. This is not required but it helps having extra nades handy. Drop the turret here as well. Wait for the doors to explode. Enemies will start coming from one side and will usually alternate sides with each wave. Kill the first wave and a half with your pistol (basically all the grunts) before using the turret on the first brute and the next enemies. You can head right up to the doors and shoot through the blue force field down the hallway to kill enemies as soon as they spawn. You can use 2-4 rockets on these waves, mainly for the 2 packs of 3-4 brutes that spawn and maybe the Jackles. Mix nades in when it feels appropriate. Once the last enemy is dead try and find a brute plasma rifle as soon as possible, otherwise grab an SMG at the turret area, along with your Rockets, or if no more Rocket ammo, your Pistol for the next fights.
On Easy difficulty the suicide grunts won't kill you by themselves so you can run into the them and let them explode while you shoot the other enemies. Be careful using stickies on the Brutes as they will often enrage and lunge at you causing the explosion to kill you. Make sure you are at a safe distance or close to full shields.
Phantoms On Your Rear Trooper
Try and get to the first wave on the left as soon as possible so that you can spawn will them with nades and/or 1-2 rockets, use 2nd weapon to clean up. If the rockets are empty try and switch them out for a Brute Plasma Rifle or a Brute shot as you head over to the second wave. Kill the remaining enemies with nades and weapons. Once all enemies are clear take this time to get as much Brute shot, find a Plasma Rifle, collect plasma/spike nades from bodies and frag nades from the walkway between the two landing areas.
Avoid meleeing brutes front on as it can be quite slow, 2-4 melees to kill. Also, since your a lightweight ODST and not a Super Space Marine the Brutes can knock you back and send you flying over the side of the building. Shoot as much as possible before going in for melees and keep and eye on your nades for any plasma or spike grenades that you may have picked up.
Elevator and Finish
Enter the elevator and hit the button (jumping over the wall is not faster as the button only becomes after a few seconds after the doors open but it looks cool). Clear the nasty buggers, or don't, on your way up. At the top walk out the doors and head around the corner. If you have plasmas or spike grenades try to stick the brute shot wielding brute before throwing 1-2 nades at the feet of the enemies landing out of the Phantom. With a few brute shots on top of the nades most/all of the non-brute enemies should die. Clear the brutes as quickly as possible focusing on the brute shot Brute, plasma rifle brutes and then any brutes closest to the centre. This is usually best done with any remaining plasmas or spike grenades you might have. Once most of the enemies are dead the Pelican will fly over and begin to descend. At this point head to the edge of the building trying to stay are full shields. If you have saved a frag and have full or close to full HP you can frag + brute shot launch up to the pelican to enter early and end the mission. Otherwise you can just frag boost or just brute shot launch up the the pelican.
It is possible to kill all the enemies before the pelican arrives so that it immediately descends all the way where you can simply jump to it. An early board only removes the need to clear every single enemy but saves a couple seconds compared to a fast clear of every enemy and simple jump to Pelican. A good line up is to head to the grating on the edge of the building, look down, throw the frag as you walk back slightly so that the frag lands close to you, then walk, jump and shoot with the frag explosion into the pelican.